Alkylaromatic compounds, such as ethylbenzene and cumene, are important intermediates in the chemical and polymer industries. For example, ethylbenzene is a commodity chemical used mostly for the production of styrene, whereas the majority of all cumene manufactured in the world today is used for the production of phenol. The demand for both ethylbenzene and cumene is rising.
Alkylaromatic compounds are typically produced by alkylating an aromatic compound, normally benzene, with an alkylating agent, typically an olefin, such as ethylene or propylene, under liquid phase or mixed gas-liquid phase conditions in the presence of an acid catalyst, particularly a zeolite catalyst. In addition to the desired monoalkylated aromatic compound, the process produces dialkylated aromatic compounds, some trialkylated compounds and other heavy by-products so it is conventional to transalkylate the polyalkylated species with benzene to increase the yield of the desired monoalkylated product. The effluent of the transalkylation reaction is then fed, together with the alkylation reaction product, to one or more benzene columns, to recover unreacted benzene, then to one or more alkylaromatic columns, to recover the desired alkylaromatic product. The unreacted benzene is recycled to the alkylator and/or transalkylator, normally after removal of water and lights (C4-hydrocarbons) either in the top section of the benzene column or in a downstream benzene lights column which may also receive makeup benzene to the plant and the vapor distillate from the benzene column(s). The bottoms of the alkylaromatic column(s) are typically further distilled in one or more polyalkylated aromatic columns to recover most of the dialkylated aromatic component and part of the trialkylated aromatic component which are sent to the transalkylator for reaction with benzene to recover additional monoalkylaromatic compound. The remainder of the trialkylated aromatic component and essentially all of the heavier compounds are typically discharged at the bottoms of the polyalkylated aromatic column as residue. An aromatic purge is sometimes taken at the overhead of the polyalkylated aromatic column to remove lower molecular weight by-products (such as butylbenzenes and cymenes in the case of cumene production, or ethyltoluenes and propylbenzenes in the case of EB production) produced in the alkylator due to the presence of reactive impurities in the olefin feed and toluene in the benzene feed.
In many existing alkylaromatic plants, particularly cumene plants, a non-aromatic purge is included as well as the aromatic purge. The non-aromatic purge can be used to remove C5, C6 and/or C7 non-aromatic impurities introduced into the system with the benzene feedstock and any non-aromatic compounds made in the alkylation and/or transalkylation reaction zones. Otherwise C5, C6 and/or C7 non-aromatics can build up in the benzene recycle loop, which increases equipment cost and energy usage. Additionally, non-aromatics that are not purged can end up in the final alkylaromatic product, where their concentration must be controlled in accordance with the specifications set by the downstream product processing. The non-aromatic purge stream typically represents a yield loss of about 0.5 wt %, may contains up to 90 wt % benzene and is usually removed as a liquid distillate at the overhead of the benzene column or when present, from the top of the benzene lights column. The purge is then normally burnt as fuel or sold to a refinery at a heavily discounted price compared to the value of the contained benzene.
According to the present invention, it has now been found that the yield loss associated with the non-aromatic purge in an aromatics alkylation process can be significantly reduced by alkylating the benzene in the non-aromatics purge in a second alkylation reaction zone, separate from the main alkylation reaction zone, to produce additional monoalkylated and polyalkylated products. After separation of the non-aromatics and the unreacted benzene, the monoalkylated product can then be used to supplement the product obtained in the main alkylation reaction zone, whereas the polyalkylated products can be converted to monoalkylated product in the transalkylation reaction zone together with the polyalkylated benzenes produced in the main alkylation reaction zone.